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isolated power for digital meter

Discussion in 'Electrical Engineering' started by James Sweet, Mar 22, 2008.

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  1. James Sweet

    James Sweet Guest

    I have a small LCD panel meter which I'd like to use in an application in
    which the power supply and the voltage being measured share the same ground.
    I know I've seen circuits before to generate an isolated voltage of 9V or so
    at a few mA for this application but can't find one at the moment.
  2. James Sweet

    James Sweet Guest

    That's not an acceptable solution, this needs to be externally powered, no
    battery to replace.
  3. If your DC voltage is at least that required to operate the LCD meter and you
    can accept a pust-to-test 'feature', then you can use a double-pole/2-position
    push button switch, resistor, a zener diode, and a capacitor.

    The capacitor goes across the common poles of the switch (watch polarity),
    the normally closed poles of the switch go to a charging circuit, and the normally
    open poles of the switch go to the LCD meter power inputs. If the voltage to be
    measured is higher than the allowed limit for the LCD power, use a series
    resistor and a zener diode across the capacitor to limit the voltage.

    The capacitor is charged when the push button switch is not pushed. When the
    push button switch is pushed, the capacitor is briefly isolated, then connected to
    the LCD meter. Years ago I used with a 250 MF 16V capacitor and each push
    would operate the meter for 10-15 seconds. Meters these days may require
    less power.

    Bill Kaszeta
    Photovoltaic Resources Int'l
    Tempe Arizona USA
  4. James Sweet

    James Sweet Guest

    That link doesn't seem to be working.

    At any rate I think you guys are making this out to be more complex than it
    really is. I saw a few circuits at one time that used a little home made
    transformer, a couple of transistors, diodes and a capacitor to form a
    little self oscillating inverter. I think I saw another one that used a 555
    and a few capacitors. It's the sort of thing I can hack together on a scrap
    of perfboard using junkbox parts. I just figured I'd ask around for a tested
    schematic rather than reinvent the wheel and design one myself. I only need
    one, this isn't something I'd have a pcb made.
  5. ehsjr

    ehsjr Guest

    A few years back I made one of those with a 2 transistor
    multivibrator with a small center tapped 600 ohm modem
    transformer as the load. A zener, diode resistor and cap
    on the output side provided the 9V.

    +---->|-----+----[R]---+--- +9
    | | |
    | [22uF] [Zd]9v
    | | |
    +-uuuuuuuuu-+----------+--- 0V
    | | | | |
    | +--------------------+--||--+ |
    | | | .1uF | |
    | | | | |
    | +----------- | -----+ |
    c\ | | | /c
    |--[10K]--+ [60R] +--[10K--|
    e/ | \e
    | o + Vin |
    | |

    Vin was tested from 6 to 16 volts and the circuit maintained
    9 volts out at 10 mA. The transformer provided the R to limit
    the current through the zener.

  6. James Sweet

    James Sweet Guest

    Ooh, I do! I hadn't even thought of that, I've got a whole pile of them.
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